The present invention relates to an image processing device used for a digital copying machine, a computer printer or a network printer and the like, and an image forming apparatus having the image processing device as an image input unit and an image processing unit.
In a large number of image forming apparatuses currently commercialized such as digital copying machines, computer printers or network printers, the electrophotography method capable of obtaining an output image of high quality at high speed as the image output unit (image output device) has widely been employed.
In the electrophotography method, as development means, there is widely used the two-component magnetic brush development in which insulating toner is charged by mixing the insulating toner with magnetic particles and causing friction in a developer unit, developer is formed in a brush shape on a development roll by means of magnetic force, and the developer is supplied on a photosensitive member by the rotation of the development roll to thereby develop an electrostatic latent image on the photosensitive member. This two-component magnetic brush development is more widely used especially for color image forming apparatuses.
However, in the image output unit of this electrophotographic type, especially in the image output unit using the two-component magnetic brush development, when two image portions having different density in the subscanning direction are continuous, there arises a phenomenon in which the density of their one image portion at the boundary portion between it and the other image portion lowers because of the non-linear and asymmetric image output characteristics in the subscanning direction.
This phenomenon has two types, one of which is that when an image outputted changes from a half tone portion 1 to a background portion 2 in a subscanning direction orthogonal to a main scanning direction which is a scanning direction of a light beam for forming an electrostatic latent image on the photosensitive member as shown in FIG. 17(A), the density at the rear end portion 1B of the half tone portion 1 in contact with the background portion 2 lowers. Hereinafter, this is referred to as decrease in the density at the half tone portion.
The other is that when an image outputted changes from a low density portion 12L to a high density portion 13H in the subscanning direction, the density at the rear end portion 12W of the low density portion 12L in contact with the high density portion 13H lowers as shown in FIG. 17(B). Hereinafter, this is referred to as decrease in the density at the low density portion.
According to the electrophotography method using the two-component magnetic brush development, as shown in FIG. 18, a photosensitive drum 310 is charged by an electrostatic latent image forming charger 320 by the rotation of the photosensitive drum 310 in a direction indicated by an arrow 311, and laser light L modulated through an image signal is irradiated onto the photosensitive drum 310 thus charged to thereby form an electrostatic latent image on the photosensitive drum 310. The photosensitive drum 310, on which the electrostatic latent image has been formed, comes into contact with a developer layer 337 on the surface of a development sleeve 335 which rotates at linear speed approximately twice the linear speed of the photosensitive drum 310 in a direction indicated by an arrow 336, whereby the toner within the developer layer 337 adheres to a latent image portion on the photosensitive drum 310 so that the electrostatic latent image on the photosensitive drum 310 is developed into a toner image.
FIG. 18(A) shows a state in the moment a latent image portion 3 of the half tone portion 1 is formed on the photosensitive drum 310 by the irradiation of the laser light L so that its front edge 3f comes into contact with the developer layer 337, FIG. 18(B) shows a state in the moment a portion somewhat on this side of the rear edge 3b of the latent image portion 3 comes into contact with the developer layer 337, and FIG. 18(C) shows a state in the moment the rear edge 3b of the latent image portion 3 comes into contact with the developer layer 337.
The developing bias at a potential of, for example, -500 V is applied to the development sleeve 335. The photosensitive drum 310 is charged at a potential of, for example, -650 V, having a higher absolute value than the developing bias potential by the charger 320, and the latent image portion 3 of the half tone portion 1 is charged at a potential of, for example, -200 V, a lower absolute value than the developing bias potential. Also, a portion 4 corresponding to the background portion 2 behind the half tone portion 1 is charged at a potential of -650 V having a higher absolute value than the developing bias potential.
When the front edge 3f of the latent image portion 3 comes into contact with the developer layer 337 as shown in FIG. 18(A), a forward development field is applied to toner tq which exists at a position Q where the photosensitive drum 310 comes into contact with the developer layer 337 so that the toner tq is brought close to the surface of the developer layer 337 to adhere to the latent image portion 3. When, however, a portion 4 corresponding to the background portion 2 behind the half tone portion 1 is brought close to the developer layer 337 as shown in FIG. 18(B), toner tb which exists in a portion of the developer layer 337 facing to the portion 4 is spaced apart from the surface of the developer layer 337 by the development field in the reverse direction to get into the depths of the developer layer 337.
By the rotation of the development sleeve 335 in the direction indicated by the arrow 336, its toner tb is brought closer to the position Q, where the photosensitive drum 310 comes into contact with the developer layer 337, and at the same time, moves to the surface side thereof because of the low potential at the latent image portion 3, but some delay occurs to reach the surface of the developer layer 337. For this reason, when the portion somewhat on this side of the rear edge 3b of the latent image portion 3 comes into contact with the developer layer 337 as shown in FIG. 18(B), the amount of toner adhering to the photosensitive drum 310 starts to decrease so that the density at the rear end portion 1B of the half tone portion 1 in contact with the background portion 2 lowers as shown in FIG. 17(A).
If the front of the half tone portion 1 is also the background portion, also when the front edge 3f of the latent image portion 3 comes into contact with the developer layer 337 as shown in FIG. 18(A), there arises, in the toner in the developer layer 337, toner which is kept away from the surface of the developer layer 337 by a portion 5 on the photosensitive drum 310 corresponding to the background portion ahead as shown by toner tf.
By the rotation of the development sleeve 335 in the direction indicated by the arrow 336, however, the toner tf is rapidly kept away from the position Q, where the photosensitive drum 310 comes into contact with the developer layer 337, and at the same time, the toner tq brought close to the surface of the developer layer 337 because of the low potential at the latent image portion 3 approaches the position Q immediately to adhere to the latent image portion 3. Therefore, even if the image outputted changes from the background portion to the half tone portion 1 on the contrary in the subscanning direction, the density at the front end portion of the half tone portion 1 in contact with the background portion does not lower.
Also, as regards decrease in the density at low density portion, FIG. 19(A) shows a state in the moment a latent image portion 32L of the low density portion 12L is formed on the photosensitive drum 310 by the irradiation of laser light L so that its front edge 32f comes into contact with the developer layer 337, FIG. 19(B) shows a state in the moment the rear edge 32b of the latent image portion 32L comes into contact with the developer layer 337, and FIG. 19(C) shows a state in the moment the latent image portion 33H of the high density portion 13H somewhat in the rear of the rear edge 32b of the latent image portion 32L comes into contact with the developer layer 337.
The latent image portion 32L of the low density portion 12L is charged at, for example, -300 V having a lower absolute value than the developing bias potential. Also, the latent image portion 33H of the high density portion 13H behind the low density portion 12L is charged at, for example, -200 V having a lower absolute value than the potential at the latent image portion 32L of the low density portion 12L.
When the front edge 32f of the latent image portion 32L comes into contact with the developer layer 337 as shown in FIG. 19(A), a forward development field is applied to toner ta existing in the position Q, where the photosensitive drum 310 comes into contact with the developer layer 337 to cause the toner ta to adhere onto the latent image portion 32L. Thereafter, until the rear edge 32b of the latent image portion 32L comes into contact with the developer layer 337 as shown in FIG. 19(B), the toner adheres to the latent image portion 32L of the low density portion 12L. The toner tc is toner adhering to the rear end portion of the latent image portion 32L in contact with the latent image portion 33H corresponding to the rear end portion of the low density portion 12L in contact with the high density portion 13H.
After the point of time shown in FIG. 19(B), however, the latent image portion 33H of the high density portion 13H will come into contact with the developer layer 337. The potential at the latent image portion 33H has a lower absolute value than the potential at the latent image portion 32L, and a forward, greater development field is applied between the latent image portion 33H and the developer layer 337, and therefore, a large quantity of toner adhere to the latent image portion 33H.
Therefore, in the vicinity of the position Q, in the developer layer 337, where the photosensitive drum 310 comes into contact with the developer layer 337, the magnetic particles which have been covered with the toner are exposed, and the potential of the magnetic particles causes the toner tc which once adhered to the rear end portion of the latent image portion 32L in contact with the latent image portion 33H as shown in FIG. 19(B) to be pulled back into the developer layer 337.
Therefore, as shown as a portion where no toner exists in FIG. 19(C) (all toner does not always disappear, but this figure shows a simplified one), the amount of toner at the rear end portion of the latent image portion 32L in contact with the latent image portion 33H decreases, and the density at the rear end portion 12W of the low density portion 12L in contact with the high density portion 13H lowers as shown in FIG. 17(B). In this respect, the amount of toner te adhering to the latent image portion 33H of the high density portion 13H becomes larger than that of the toner ta adhering to the latent image portion 32L of the low density portion 12L, but FIG. 19(C) shows as the same quantity for convenience sake.
Decrease in the density at the rear end portion 12W of the low density portion 12L, that is, reduction of the amount of toner at the rear end portion of the latent image portion 32L is caused by the toner tc adhering to the rear end portion of the latent image portion 32L being pulled back into the developer layer 337 by a potential having a low absolute value, at the latent image portion 33H of the high density portion 13H immediately subsequent to the low density portion 12L, and therefore, the density at the front end portion of the low density portion in contact with the high density portion does not lower even if the image outputted changes from the high density portion to the low density portion on the contrary in the subscanning direction.
Thus, in the electrophotography method using the two-component magnetic brush development, when an image outputted changes from the half tone portion to the background portion in the subscanning direction, the density at the rear end portion of the half tone portion in contact with the background portion lowers, and when an image outputted changes from the low density portion to the high density portion in the subscanning direction, the density at the rear end portion of the low density portion in contact with the high density portion lowers,
In Japanese Published Unexamined Patent Application Nos. 5-281790 and 6-87234, there is shown an idea that the accuracy of a laser light scanner for writing an electrostatic latent image on a photosensitive member through laser light is improved, and the parameter of development means for developing the electrostatic latent image is adjusted to thereby enhance the contrast of the development field for preventing decrease in the density at the half tone portion and decrease in the density at the low density portion described above.
However, the method of enhancing the contrast of the development field by improving the accuracy of the laser light scanner, which is the means for writing an electrostatic latent image, leads to a large size and a high cost of the image output unit. And yet, if the number of screen lines is increased at the image output unit to improve the resolution of the output image, the contrast of the development field lowers to more easily cause decrease in the density at the half tone portion and decrease in the density at the low density portion. Therefore, it is difficult to avoid a large size and a high cost of the image output unit while achieving higher resolution of the output image by an increase in the number of screen lines.
In recent years, with the spread of computer printers and network printers, an opportunity of printing a graphic image formed on a host computer such as a personal computer tends to increase. In such a graphic image, decrease in the density at the half tone portion and decrease in the density at the low density portion more easily attract people's attention than a natural image such as photographs. Therefore, in an image forming apparatus such as computer printers and network printers, decrease in the density at the half tone portion and decrease in the density at the low density portion pose a more serious problem than in an image forming apparatus such as copying machines.
As a method for correcting such output characteristic of an image forming apparatus in a linear and symmetrical area having high spatial frequency as MTF characteristic, a method for correcting input image data using a digital filter process has widely been employed.
However, the aforesaid lowered density at the half toneportion and lowered density at the low density portion occur non-linearly and asymmetrically only in the image portion in front of the image edge in the subscanning direction, and the area extends over a range of 1 to 2 mm, although it depends upon the pixel value of the half tone portion in front of the image edge or the difference in pixel value between low density portion and high density portion before and behind the image edge. Also, in order to, determine whether or not decrease in the density occurs, and the characteristic properties such as range and amount of decrease in the density if it occurs, the image data after the edge must be also observed over the same extent of range.
To that end, if an attempt is made to reduce or prevent decrease in the density by the digital filter process, the process extends over such a wide range as 4 mm in the subscanning direction, for example, 64 pixels continuous in the subscanning direction in an image forming apparatus having resolution of 16 dpm (dot/mm), 400 dpi (dot/inch) or 96 pixels continuous in the subscanning direction in an image forming apparatus having resolution of 24 dpm, 600 dpi. In addition, since decrease in the density is non-linear and asymmetric, and moreover occurs in an area with low spatial frequency, it is impossible to reduce or prevent decrease in the density by the digital filter process.
As a method for preventing decrease in the density at the half tone portion and decrease in the density at the low density portion, the inventors considered a method to avoid a large size and a high cost of the image output unit while achieving higher resolution of the output image by an increase in the number of screen lines.
This is to detect a half tone portion or a low density portion, in which decrease in the density occurs, from the input image data to thereby correct the pixel value at the half tone portion or the low density portion of the input image data so as to supplement the amount of lowered density. In this case, the characteristic properties such as the range and amount of decrease in the density occurring in the image output unit are elucidated or measured in advance to obtain correction characteristic corresponding to the density decrease characteristic. Thus, the correction characteristic is described in the image processing device to be furnished for correction of the input image data during image processing.
As a concrete method, it is conceivable to write the image data in a page memory after correcting it. More specifically, in the case of a digital copying machine, as shown in FIG. 20(A), image data Ri, Gi and Bi of red, green and blue from the image input unit 100 are supplied to a color conversion gradation correction unit 201 in the image processing unit 200, and image data Yi, Mi, Ci and Ki of yellow, magenta, cyan and black can be obtained from the color conversion gradation correction unit 201.
Thus, the image data Yi, Mi, Ci and Ki from the color conversion gradation correction unit 201 are supplied to an image correction unit 202, in which the pixel values for the image data Yi, Mi, Ci and Ki are corrected, and the image data after the correction are written in the page memory 203. Then, after the image data for one page are written, the image data Yo, Mo, Co and Ko of yellow, magenta, cyan and black are read as the output image data from the page memory 203 to be fed to an image output unit 300.
In this case, in a copying machine extending over the aforesaid range of about 4 mm in the subscanning direction, that is, having resolution of 16 dpm, 400 dpi as shown in FIG. 20(B), the image correction unit 202 processes image data in the main scanning line Lm extending over about 64 lines continuous in the subscanning direction simultaneously to detect the half tone portion 1 and the background portion 2 or the low density portion 12L and the high density portion 13H from an area 20 before and behind the image edge Eg, to detect the characteristic properties of decrease in the density at the half tone portion 1 or the low density portion 12L, and to correct the pixel value retroactively to the image data before the image edge Eg referring to the correction characteristic described in the image correction unit 202.
In the case of writing the image data in the page memory 203 after correcting them by the image correction unit 202 in this way, however, it is necessary to store as an enormous amount of pixel data as hundreds of thousands extending over several thousands of pixels in the main scanning direction and extending over such many main scanning lines Lm as 64 lines in the subscanning direction in a correction memory separate from the page memory 203 for two dimensional image processing, thus leading to an immense circuit capacity of the image correction unit 202 and a higher cost of the image processing unit 200.
Thus, as another method, it is conceivable to correct the image data after writing them in the page memory. More specifically, in the case of a digital copying machine, as shown in FIG. 21(A), the image data Yi, Mi, Ci and Ki from the color conversion gradation correction unit 201 for each page are written in the page memory 203, and thereafter, the image data written in the page memory 203 are corrected by the image correction unit 202. After the correction, the image data Yo, Mo, Co and Ko are read out as output image data from the page memory 203 to be fed to the image output unit 300.
In this case, the image correction unit 202 simultaneously processes the image data of, for example, 64 pixels continuous, on one of lines Ls in the subscanning direction as shown in FIG. 21(B) to detect the half tone portion 1 and the background portion 2 or the low density portion 12L and the high density portion 13H from the area 20 before and behind the image edge Eg, to detect the characteristic properties of decrease in the density at the half tone portion 1 or the low density portion 12L, and to correct the pixel value retroactively to the image data before the image edge Eg referring to the correction characteristic described in the image correction unit 202. This operation is repeated for each line Ls in the subscanning direction.
Therefore, in this case, the processing scale becomes noticeably smaller than in the case of writing the image data in the page memory 203 after correcting them by the image correction unit 202 as shown in FIG. 20. In this case, however, there is inconvenience that data cannot be corrected on a real-time basis because the image data are corrected after image data for one page are written in the page memory 203.
In the case of a network printer, it is also conceivable to correct the image data after they are written in the page memory as described below. More specifically, in the case of the network printer, there is taken in an image processing unit 700 print information described in page description language (hereinafter, referred to as PDL) transmitted from a client apparatus consisting of a computer, a work station and the like, which are omitted in the figure, to the network 400 by a communication control unit 710 in the image processing unit 700 as shown in FIG. 22.
In the image processing unit 700, a communication protocol analytical control unit 721 analyzes the protocol of the input information, a PDL command/data analysis unit 722 analyzes the print information described in PDL, that is, PDL command/data, and the image development unit 770 develops the image data as a data row in the main scanning direction. If code data from the PDL command/data analysis unit 722 contain character information, the image data concerning characters are developed by the outline information from a character development unit 724.
Also, a color determination unit 725 produces a parameter for converting the image data developed in the image development unit 770 into image data for each color of YMCK on the basis of color information of the PDL command/data. The parameter causes an information association unit 726 to convert the image data developed by the image development unit 770 into image data each color of YMCK.
The image data for each color of YMCK from this information association unit 726 are written by an amount corresponding to one page in the page memory 760, and thereafter, the image data written in the page memory 760 are corrected by a correction painting unit 780. After the correction, the image data for each color of YMCK are read out as the output image data from the page memory 760 to be sent to the image output unit 800. The image data are corrected at the correction painting unit 780 by reading the image data in the subscanning direction from the page memory 760 in the same manner as in the image correction unit 202 in the case of the digital copying machine shown in FIG. 21.
In this case, however, there is inconvenience that data cannot be corrected on a real-time basis because the image data are corrected after the image data for one page are written in the page memory 760 in the same manner as in the case of the digital copying machine shown in FIG. 21.
Also, in the case of a network printer, compressed image data referred to as intermediate code are generally written in the page memory 760 from the image development unit 770 through the information association unit 726. When, however, the compressed image data are thus written in the page memory 760, all compressed image data in the main scanning line must be analyzed to read the image data in the subscanning direction from the page memory 760 for correction in the correction painting unit 780, thus resulting in a complicated process. To that end, when an attempt is made to write image data in an uncompressed form in the page memory 760, there arises inconvenience that a large capacity of page memory 760 will be required.
From the foregoing, the present invention prevents decrease in the density due to the image output characteristic of the image output unit in the subscanning direction by means of a method to avoid a large size and a high cost of the image output unit while achieving higher resolution of the output image by an increase in the number of screen lines, and real-time processing without causing the circuit capacity and the memory to be increased.